Severe joint pain

I've been on Imuran since Jan 6, 2009, and it appears to be slowly helping my Crohn's. Blood tests get better every two weeks, and the GI doc has slowly decreased my Prednisone down to 30mg a day (it was 80 after my flare up in December). But two days ago, I started having pain in my knees in the evening. It worsened over the night, finally becoming so unbearable that I had to go to ER at 5am. The pain was in both knees, some bit also in ankles, and tylenol did not touch it. I seem to remember that a possible side effect of Imuran is joint pain. Is this correct? At the ER, blood tests showed me to be low in Magnesium, otherwise normal (for my Crohn's state). After some pain meds, they sent me home and today went as normal. Has anyone else had this occur? Thanks in advance.
Pigdoc

currently: pred 30mg/d, Pentasa (1000mg/d), Imuran (4.5 pills/d), immodium (1/d), iron pill (3/d), Lantasa (42units/d), Humalog (as blood test dictates, about 4 units/d)

i'm not on imuran, but i have joint /muscle pain on and off....sometimes really bad...never bad enough to go to the hospital though...could be the prednisone also i think...i'm at 40 right now tapering from 60

Thanks for the response. As to weight, I'm down to 185, still heavy for 5'10", but not huge. I'll check into the folic acid and B12 areas.